Spitting out that contributing upstream feels like it is not worth your effort, even if it would help everyone, because you can't use it for years due to supporting ancient versions is one of the reasons why WP is seen as holding PHP back.
I've only ever seen WP developers use Rector to *downgrade* their PHP code as they want to write modern code. Nobody asked you to maintain PHP 5.2 compat in 2019 (for WP 5.1). For context this means no namespaces, as this is a PHP 5.3 feature.